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VCO Oscillation frequency

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Mahruz

Junior Member level 3
Hi All,

I am trying to find what the oscillation frequency is for the below colpit oscillator circuit.
Because of the varactors combination in terms of its bias, it is difficult for me to understand what its capacitance is.

I have also linked the datasheet for the varactors, transistor and resonator:
Q2--> https://www.digikey.com.au/en/products/detail/cel/2SC4226-T1-A/5218749
D50, D53 --> https://au.mouser.com/ProductDetail/Toshiba/1SV229TPH3F?qs=EEns8I54Y6DXAflo8LWGLA==
CR --> http://www.trans-techinc.com/webcoa...400&ftol=b&tab=yes&subInquiry=Find+Resonators
For the CR; please use the information from the second image below marked with red circles
The voltage at L10 is usually in the range of 0.5V to 3.3V(not sure if this is helpful)

The Frequency of Oscillation essentially is determined by Coaxial Resonator. Varicaps are used to shift slightly the Frequency if there is a tolerance. But the CR plays the major role on that.

Hi Big boss,

How can I calculate the frequency it is oscillating at?

For example; I want the CR, varicaps and series caps, all combined together to oscillate at 860MHz. Is there some way to simplify the circuit ?

Thanks

Hi Big boss,

How can I calculate the frequency it is oscillating at?

For example; I want the CR, varicaps and series caps, all combined together to oscillate at 860MHz. Is there some way to simplify the circuit ?

Thanks
For that frequency, it's difficult to find a Coaxial Resonator due to Electrical Length.
There may be some custom designed resonators for different purposes but it's hard to find.
If the manufacturing volume is sufficiently high, a custom design may be requested but it will cost you.
If you're able to access to RF&Microawave Simulator like ADS or AWR, you can put equivalent circuits of CR and Varicaps and you do an optimization. But oscillators are chaotic systems therefore you should design an appropriate PCB to test.
It's principally a Clapp-Gouriet oscillator and I recommend you read Rohde's Textbook to understand the key point of the oscillators.

oscillate at 860MHz

Formula to determine frequency given L & C values (√ means square root):
f= 1/ (2π√(LC))

Oscillations are easier to sustain if the L:C ratio is in a range from 1,000 to 100,000.

I'm running a simulation of a Clapp oscillator. For reasonable values I get 10nH for a single inductor, and...
10pF each for two or three capacitors in series with the inductor.

Commonly a Colpitts has two caps in series. The Clapp has three caps.

For that frequency, it's difficult to find a Coaxial Resonator due to Electrical Length.
There may be some custom designed resonators for different purposes but it's hard to find.
If the manufacturing volume is sufficiently high, a custom design may be requested but it will cost you.
If you're able to access to RF&Microawave Simulator like ADS or AWR, you can put equivalent circuits of CR and Varicaps and you do an optimization. But oscillators are chaotic systems therefore you should design an appropriate PCB to test.
It's principally a Clapp-Gouriet oscillator and I recommend you read Rohde's Textbook to understand the key point of the oscillators.
What is the full name or link of the book?
--- Updated ---

Formula to determine frequency given L & C values (√ means square root):
f= 1/ (2π√(LC))

Oscillations are easier to sustain if the L:C ratio is in a range from 1,000 to 100,000.

I'm running a simulation of a Clapp oscillator. For reasonable values I get 10nH for a single inductor, and...
10pF each for two or three capacitors in series with the inductor.

Commonly a Colpitts has two caps in series. The Clapp has three caps.
Hi,

I am aware of the formula,but not sure what the capacitor and inductor value should be used to find the frequency.
This is where I am confused.

What is the full name or link of the book?

not sure what the capacitor and inductor value should be used to find the frequency.
You might as well make it easy on yourself and assume the resonant action runs equally through all capacitors. This means my three 10pF series caps act like a single 3.3 pF.

Your schematic is complicated, making it hard to evaluate how the frequency arises from multiple inductors and capacitors (and then there are varactors too).

Why are you choosing a 1400 MHz resonator when you want 860 MHz oscillator frequency? I doubt that can tune the 1400 MHz resonator down to 860 MHz and achieve good oscillator performance.
--- Updated ---

I notice that the schematic in post #1 still shows a questionable varactor configuration where D50 has floating DC bias.

Last edited:

Why are you choosing a 1400 MHz resonator when you want 860 MHz oscillator frequency? I doubt that can tune the 1400 MHz resonator down to 860 MHz and achieve good oscillator performance.
--- Updated ---

I notice that the schematic in post #1 still shows a questionable varactor configuration where D50 has floating DC bias.
1400Mhz as not chosen by me. It was done by a previous engineer and I wanted to understand these aspects.
In terms of the floating bias for D50, can you please explain.

Thanks,

Please check how the voltage across D50 is determined in the shown assembly variant.

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